1. Technical Field
The present invention relates to a distance measuring apparatus for measuring distances using laser beams, and the present invention specifically relates to a distance measuring apparatus in which two kinds of laser beams, having different wavelengths, are simultaneously output.
2. Background Art
As a technique for measuring distances using a laser beam, a technique of using two kinds of laser beams, having different wavelengths, is known. For example, Japanese Unexamined Patent Application Publication No. 9-318743 discloses a structure in which two kinds of laser beams, having different wavelengths, are combined and emitted at an object. In this technique, a target object is identified according to an intensity ratio of the two wavelengths of reflected light, and the distance to the object is calculated from the response time of the reflected light. The above reference discloses a structure in which a laser oscillator for two kinds of lasers, having different wavelengths, is provided in an apparatus, and these two kinds of lasers are appropriately combined or switched in an optical system.
There are the following problems in measurement of a distance using a laser beam. The first problem is that reflection intensity of a target object depends on the wavelength of light. The reflectivity depends on the wavelength according to the color and the material of the target object. Therefore, light may be reflected at low intensity when a combination of a target object and the wavelength is inappropriate, and the reflected light is thereby not easily detected.
The second problem is the influence of the measurement environment. That is, a laser beam having a shorter wavelength scatters and is greatly attenuated when there is fog or smog present.
The third problem is safety for the eyes. Light having a wavelength of not more than that in the visible light range may be transmitted through the eyeball and may be very easily absorbed by the eyeground. Therefore, light may have a deleterious effect on the eye when the intensity thereof is high.
For example, since light having an infrared wavelength of not less than 1400 nm is not substantially transmitted through the cornea, the crystal lens, and the corpus vitreum and is not substantially absorbed by the eyeground, a high-power laser beam can be used safely. Therefore, a laser beam having an infrared wavelength of not less than 1400 nm is called an “eye-safe laser”. On the other hand, a laser beam having a wavelength of around 1000 nm has a transmission rate of approximately 40% with respect to the crystal lens, and the like and has an absorption rate of approximately 10% with respect to the eyeground. Therefore, the laser beam may have a deleterious effect on the eye when the power thereof is high. A laser beam having a wavelength of around 500 nm has a transmission rate of approximately 90% with respect to the crystal lens, and the like, and has an absorption rate of approximately 70% with respect to the eyeground. Accordingly, such a laser beam may have worse effects on the eye.
The fourth problem is that light having a long wavelength is useful for measuring long distances, but is inferior in measurement resolution because diffraction spread is large. On the other hand, light having a short wavelength is superior in measurement resolution, but is not useful for measuring long distances because the light may easily scatter.
As a method for solving these problems, a method using laser beams having different wavelengths may be mentioned. Japanese Unexamined Patent Application Publication No. 9-318743 discloses a structure comprising laser oscillators with respect to each wavelength. Since such a structure is complicated, the apparatus may not be of compact design, and the production cost cannot be reduced.